INFLUENCE OF PHASE-TRANSFORMATION ON DENS IFICATION BEHAVIOR AND GRAIN-GROWTH OF FINE SILICON-NITRIDE POWDER

Fine and uniform silicon nitride powders having different or-content w ere prepared from commercial submicron powders by centrifugal sediment ation, The densified pre-sintered materials were fabricated from these powders by hot-pressing. The fine powders with similar particle size distribution showed the same densification behavior during hot-pressin g. The microstructures of pre-sintered materials of the fine powders s howed the fine and homogeneous microstructures, and the similar one ir respective of alpha content, The pre-sintered materials were annealed at 1800 degrees C for 1h under 1 MPa nitrogen atmosphere. The annealed material from the fine beta powder showed the fine and homogeneous mi crostructure. On the other hand, the material from the fine alpha powd er developed ''in-situ composite'' microstructure consisting of large grains and fine matrix grains by annealing, Because, the abnormal grai n growth was caused by the alpha- to beta- phase transformation during annealing. While, these results of the fine powders were compared wit h those of the commercial submicron powders.